1. Field of the Invention
The present invention relates to a steel pipe drawing apparatus and a drawn pipe manufacturing method, and in particular to a steel pipe drawing apparatus and a drawn steel pipe manufacturing method for manufacturing a drawn steel pipe by moving a die and a plug relatively in the same direction as a steel pipe drawing direction and in an opposite direction thereto.
2. Description of the Prior Art
Conventionally, a hollow shaft with a desired wall-thickness has been preferably used for a boring rod for resource development, a shaft of an automobile or the like in view of reduction in weight and reduction in materials cost. The hollow shaft served for these applications is formed from a stepped drawn steel pipe with a plurality of outer diameters and a plurality of inner diameters formed along a longitudinal direction. Such a drawn steel pipe is manufactured by cold-drawing a material steel pipe using a die and a plug generally (for example, see Patent Literature 1 and Patent Literature 2). According to such a drawn steel pipe manufacturing method, by drawing a material steel pipe while pressing it between the die and the plug using a steel pipe drawing apparatus, a drawn steel pipe with a desired size is manufactured. At this time, by changing a bearing diameter of the die and a bearing diameter of a tap properly according to a drawing position of the material steel pipe, a stepped drawn steel pipe with a plurality of outer diameters and a plurality of inner diameters formed along the longitudinal direction can be manufactured.    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 59-73113    Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 59-73115
In the above conventional steel pipe drawing apparatus, however, a moving velocity of a hydraulic cylinder is constant regardless of a cold-drawing velocity. In other words, in the conventional steel pipe drawing apparatus, because the drawing velocity is constant regardless of change of a wall-thickness of the drawn steel pipe, dimension accuracy of a portion whose wall-thickness changes in a stepped drawn steel pipe is lowered. Specifically, when the material steel pipe is drawn, sags occur in a wall-thickness gradient in a second wall-thickness changing region in which the material steel pipe is drawn such that the wall-thickness becomes thicker in comparison to a wall-thickness gradient of a first wall-thickness changing region in which the material steel pipe is drawn such that the wall-thickness becomes thinner, which results in that dimensional accuracy of the second wall-thickness changing region is extremely lowered in comparison to that of the first wall-thickness changing region. As a result, there occurs such a defect that cutting positions of a continuous drawn steel pipe which is a long object cannot be managed with high accuracy.
Further, since the conventional steel pipe drawing apparatus is not computer-controlled, a dimensional tolerance of a drawn steel pipe cannot be improved, which results in lowering of a grade average of drawn steel pipe products. Furthermore, since the number of moving strokes of the hydraulic cylinder is set to a few times (four, for example), the whole length of a continuous drawn steel pipe manufactured by the steel pipe drawing apparatus is restricted. In other words, one stroke is a stroke for forming a drawn steel pipe corresponding to one piece produced by moving the hydraulic cylinder once back and forth, and a serial drawn steel pipe with a size corresponding to four pieces is manufactured by four strokes, but in the case of a drawn steel pipe which is a long object with a size corresponding to about four pieces, there occurs such a defect as increase in dimensional tolerance in the longitudinal direction. Therefore, in the conventional steel pipe drawing apparatus, in view of maintaining dimensional accuracy in the longitudinal direction of the drawn steel pipe with high accuracy, the number of the moving strokes of the hydraulic cylinder cannot be increased more than about four times. In other words, in the conventional steel pipe drawing apparatus, a drawn steel pipe which is a long object cannot be manufactured with high accuracy.
Therefore, there occurs technical problems which must be solved in order to achieve a steel pipe drawing apparatus and a drawn steel pipe manufacturing method which can manufacture a drawn steel pipe which is a long object with high accuracy and at high speed as well as improve dimensional accuracy of respective wall-thicknesses portions and cutting positions of a drawn steel pipe, and an object of the present invention is to solve the problems.